Nicholas Revel (Peter Lawford) is a young man any dowager in London welcomes to her parties. He has taking ways, and the Calgurie Emerald is one of the things he takes. This daring theft brings unexpected complications, amongst them the daughter, Jane Frensham (Dawn Addams), of the head of Scotland Yard, Sir Herbert Frensham (Sir Michael Hordern) and a maniacal killer loose in London. Revel sets a trap for the madman, with himself as bait, in a fog-shrouded stalk where one of them has to die.